The present invention relates to an optical scanning device.
Optical scanners are used in a wide variety of applications to convert information that appears as changes in light intensity into electrical signals and vice versa. The information may be in analogue form but one of the most demanding applications is the recordal and retrieval of digital data.
The recording and retrieval of data on an optical media in digital form provides significant advantages in terms of data storage density, permanence and retrievability. The most common form of data storage using digital optical recording techniques utilizes a record media in the form of a disk that is rotated beneath an optical head to record and/or read data on the media. This arrangement has enjoyed considerable success, particularly in the audio field, but does pose limitations in terms of the media format.
An alternate format that offers significant advantages in certain application areas is a card format in which the media is held stationery and the optical beam reading the media is moved or scanned across the media. This allows different formats and sizes of media to be used, most conveniently a format corresponding to that of the popular size of credit card.
Similarly, a continuous tape offers advantages in certain applications such as archival recording and very high data storage capabilities.
In order to attain high data rates and data densities, it is preferable to utilize a rotatable scanner that causes an optical beam to translate relative to the record to read and record data. Such devices are disclosed in U.S. Pat. Nos. 3,501,586 issued Mar. 17, 1978; 3,624,284 issued Nov. 30, 1971; and 3,795,902 issued Mar. 5, 1974 all to Russell. In German Offenlegungshrift DOS 2,022,265 issued Nov. 18, 1971 published to Siemens, a scanning device is disclosed that utilizes a multi-faceted pyramid rotated about its central axis that serves to displace radially a light beam to an objective lens to scan a media. The device disclosed in this patent provides a simple yet effective distribution apparatus but as higher data density is required, conflicting mechanical and optical requirements introduce undesirable error in the scanning process. As the planar facet of the pyramid rotates, the point of impingement of the light beam on the face of the pyramid varies, causing the optical axis of the light beam to move across the aperture of the objective lens. This causes the beam to illuminate the outer zone of the lens during part of the scan with a resultant loss in quality of the optical system.
U.S. Pat. No. 4,163,600 issued Aug. 7, 1979 to Russell addresses these problems by introducing a back angle between the light beam and the pyramidal mirror that reduces movement of the light beam across the objective lens. A further approach to reducing the movement of the optical axis relative to the lens is shown in co-pending application Ser. No. 162,299 now U.S. Pat. No. 4,861,144 in which a field correction lens is used to generate a virtual image co-incident with the axis of rotation of the scanner. Each of these proposals does produce benefits but nevertheless at the high density and data rates required for optical storage devices, a simpler approach is preferable.
It is therefore an object of the present invention to provide an optical scanning device in which the above disadvantages are obviated or mitigated.